Composite video signals of the type used in television systems include video and synchronizing information. The video information is divided into sequential trace and retrace signals with the synchronizing information transmitted as pulses during the retrace portion of the video information. During the trace interval, the video signal varies between black and white levels. The retrace interval video signal includes a blanking portion which is at or slightly greater than black level so that the electron beam in the cathode ray tube (CRT) is turned off during retrace. If all of the levels of the various components of the video signal are properly adjusted at the studio and properly transmitted and received, the video signals can be properly displayed on a CRT screen. Unfortunately, adjustment errors and transmitting and receiving errors require processing and correction of the video signal to achieve a satisfactorily displayed image.
A form of video signal error which commonly occurs is degradation of the sharpness of the image. Numerous variations in picture sharpness and numerous causes for such variations are encountered in typical TV transmissions. Such variations can be caused, for example, by differing video signal transient characteristics of the various video signal sources used in the TV studio. Also, the video distribution systems used in the studio may have different video frequency response and group delay characteristics. Transmitters and modulators at different studios or stations also have different video frequency response and group delay characteristics. Multipath or ghost pick-up conditions at the receiving antenna can greatly vary detected video signal transient response due to signals being cancelled at some frequencies and reinforced at others. Additionally, the TV receiver radio frequency intermediate frequency, and video responses may vary from channel to channel. Factors other than those mentioned above may also cause variations to picture sharpness.
Two somewhat related forms of compensation for variations in picture sharpness are known in the prior art. One form is generally called video peaking which is a form of high frequency emphasis. Typical prior art video peaking circuits boost the high frequency components of the video signal. Such circuits also typically have an undesired phase delay associated therewith.
The term "aperture correction" is primarily used with cameras to refer to compensation for spot size. The term is also used in receivers to refer to a particular form of video peaking which does not have an undesired phase delay characteristic associated therewith. Aperture correction involves the addition of pre-shoot and over-shoot components to transitions or transients in the video signal. Thus, the sharpness of the displayed image is enhanced by sharpening the transition from one brightness level to another brightness level.
A typical prior art aperture correction circuit includes a differential stage with a delay line connected between the two inputs. The video signal is applied to the first input with a delayed video signal applied to the second input. These two signals cause the differential stage to provide a pre-shoot signal. The receiving end of the delay line is unterminated so that a reflected and twice delayed video signal is also applied to the first input. The delayed and twice delayed video signals cause the differential stage to provide an over-shoot signal. The pre-shoot and over-shoot signals are added to the delayed video signal to provide an aperture corrected video signal.
While numerous forms of aperture correction and peaking circuitry are known in the prior art, such circuitry typically suffers from one or more disadvantages. Discrete aperture correction circuitry is typically simple to avoid expense but has less than optimum performance. Other known forms of aperture correction circuitry are unduly complex. In addition, known prior art aperture correction circuitry tends to emphasize noise thereby deleteriously affecting the weak signal performance of the television receiver.
It is known in the prior art to de-emphasize the high frequency video signal components in response to increased automatic gain control (AGC) or other similar measures of signal strength. Such depeaking circuits, however, do not operated directly in response to high frequency signal components. Thus, such circuits do not correct for errors which affect high frequency video signal components differently from the low frequency video components.